Fibre optic sensing is known for a variety of different applications. Fibre optic sensors typically operate by interrogating an optical fibre with optical radiation and analysing any radiation backscattered, either from deliberate point sensors within the fibre (e.g. Fibre Bragg gratings or the like) for from intrinsic scattering sites within the fibre itself, to determine various parameters such as strain, vibration or temperature.
One type of fibre optic sensing is fibre optic distributed acoustic sensing (DAS) wherein the optical fibre is interrogated to provide sensing of acoustic activity along its length. Typically one or more optical pulses are launched into the fibre and the radiation backscattered from within the fibre is detected, i.e. backscatter from intrinsic scattering sites inherent in the fibre rather deliberately introduced discrete reflectors. The detected backscattered radiation is analysed. The analysis effectively divides the optical fibre into a plurality of discrete sensing portions. Within each discrete sensing portion mechanical disturbances of the fibre, for instance due to incident acoustic waves, cause a variation in the properties of the radiation which is backscattered from that portion. This variation is detected and analysed and used to give a measure of the intensity of disturbance of the fibre at that sensing portion. GB2,442,745 describes one example of a DAS system. As the radiation which is detected any analysed is scattered from the intrinsic scattering sites in the fibre the scattering, and hence the sensing function, is distributed along the whole length of the fibre.
DAS has been proposed for a variety of different applications. For instance it has been proposed to use fibre optic DAS for perimeter security, using a sensing fibre optic deployed along the route of the perimeter of interest to monitor for acoustic signals associated with intruders. It has also been suggested to use fibre optic sensors for structural monitoring by embedding optical fibres within the medium of the structure to be monitored.
DAS thus relies on incident pressure waves/vibrations acting on the optical fibre so as to alter the characteristics of the backscatter from the fibre. The response of a DAS sensor to a given stimulus may however be dependent on the environment in which the optical fibre is deployed and the coupling of the optical fibre, which will generally be in some sort of fibre optic cable structure, to the surrounding environment.
In some applications it would be useful to determine some characteristics of the environment in which the optical fibre is deployed. Embodiments of the present invention therefore provide fibre optics sensors able to determine some characteristic of the environment in which the sensing fibre is deployed.